For Sean Shaynak, the talented teenager whose high school classmates thought he could have been anything, nothing ever really stuck.

Not the first two colleges he attended. Not the pilot’s job with ExpressJet. Not the relationship with the mother of his child, the flight attendant 13 years his junior and just out of high school when they started dating. Not their charming house in Maryland. He rolled through life, from city to city, layover to layover, managing to avoid many of the responsibilities that adulthood usually delivers. Then he decided to return home to New York and go back to high school. This time, he would be a teacher.

Over the five years he was at Brooklyn Technical High School, Mr. Shaynak put down roots. He became the kind of teacher many students dreamed of having, concerned and engaged. But he also never seemed to grow up, they said. He said “LOL” and “for realsies,” and let students call him “ShayShay.” At 44, Mr. Shaynak appeared forever stuck in high school.

“He acts like a teenager,” said Yana Krasnokutskaya, 18, who transferred out of Brooklyn Tech last year. “He, in essence, is a teenager in an old man’s body,” she said, adding, “He’s very good at passing off inappropriate things as being appropriate.”

Mr. Shaynak, an aerospace and physics teacher who still acted the part of a pilot in school — even wearing his old uniform there — was accused last week of abusing seven teenage girls over three years, of 36 charges including sexual abuse, forcible touching, kidnapping, endangering the welfare of a child and disseminating indecent material. Mr. Shaynak, who pleaded not guilty to the charges, is scheduled to next appear in State Supreme Court in Brooklyn on Nov. 25.

Interviews with dozens of people who knew him yielded few clues that Mr. Shaynak might be capable of this kind of predatory behavior. They painted a picture of a brilliant but flawed man, respected by almost all who knew him — at least when they knew him, before his students became his main social network. In high school, he had a leading role in the musical “Damn Yankees,” with a voice that “pulled many heart strings,” his yearbook said. He was the aviation student so good that his instructor asked him to help teach a “principles of flight instruction” course. He was the pilot so meticulous he would rather leave his job than cut any corners.

“I looked up to him,” a former high school classmate said, speaking on the condition of anonymity for fear of retaliation. “I remember thinking that he was very talented, just in general. I would have thought he’d gone to an Ivy League school. He was an exceptional student. I saw him as an exceptional person.”

But there were signs of trouble. He had a temper, once attacking an 11-year-old neighbor for throwing rocks, the neighbor’s family said. He was disciplined for berating a student in 2012. He courted instability. He did nothing to save his Maryland home from foreclosure. He ignored a landlord’s attempts to force him to pay long-overdue rent on his apartment in the Washington Heights section of Manhattan.

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Mr. ShaynakCreditByron Smith

As a new teacher, Mr. Shaynak seemed conflicted about where to draw lines with students. He showed up to a high school party one year, and then told a student the next year that attending students’ parties was a bad idea, the student said. He told some students he could not be friends with them on Facebook until they graduated, but he made exceptions for others. Despite his apparent reservations, he rationalized going to students’ parties, saying he only wanted to listen to new music. He regularly smoked outside the school with students.

Mr. Shaynak had little success with relationships. In high school and in flight school, he occasionally dated, but nothing serious, friends and acquaintances said. His one long-term relationship, with the mother of his daughter, started when he was a pilot and she worked at ExpressJet, a year after she joined the airline. She was just out of high school, the flight attendant’s father said.

The mother, now 31 and living in a small town in upstate New York, said she was keeping their 9-year-old daughter’s knowledge of the case to a minimum and declined to comment. The woman’s father, who agreed to speak on the condition that only his first name, Jerry, be used, said he thought the age difference was a little weird, but he respected his daughter’s choice of Mr. Shaynak. “He always came across as just a very upstanding and well-mannered gentleman,” Jerry said.

Neither Mr. Shaynak nor his family would comment, his lawyer, Kimberly Summers, said.

After high school in Northport, N.Y., Mr. Shaynak, who had been a member of the National Honor Society and a choir member, struggled to find his way.

Mr. Shaynak spent some time at the University of Richmond in Virginia, studied to be a pilot on the side, and then gave New York University a try for a year. In October 1993, he moved out to Chicago, following his brother, Craig, an actor. After three years teaching at a flight school, he moved back to New York, joining SUNY Farmingdale’s professional pilot program and earning his bachelor’s degree in 1998.

Nine months after graduating, Mr. Shaynak was hired by ExpressJet, operating as Continental Express, then the regional subsidiary of Continental Airlines. Within a couple of years, he became a “check airman” — a prestigious job that involved making sure new pilots did everything right.

“He was absolutely — and this is what makes this current situation so unbelievable — he was always meticulous about doing things the right way,” said Phillip Poynor, who taught Mr. Shaynak flight instruction at SUNY Farmingdale.

ExpressJet declined to verify Mr. Shaynak’s employment. ExpressJet pilots who remembered him talked in an online chat room about Mr. Shaynak flying out of Newark.

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Mr. Shaynak’s yearbook photo from Northport High School in Northport, N.Y., where he was in the choir.CreditBryan Thomas for The New York Times

In early 2003, Mr. Shaynak bought a new house in Havre de Grace, Md., near his sister Ariel. It was a major step for him. Before, he had moved from one place to the next to his parents’ home. Within a couple of years, Mr. Shaynak’s girlfriend and their baby joined him, neighbors said.

The new life was not always peaceful. In October 2005, neighbors sought a restraining order against Mr. Shaynak, accusing him of attacking their 11-year-old son who had thrown rocks at his house. Neighbors said Mr. Shaynak feared his daughter would be hurt by broken glass.

About 18 months after moving in, the woman and their daughter moved out of the house.

This incarnation no longer worked for Mr. Shaynak. He was proud of being a pilot, but the job wore on him. He grumbled about his long commute to New Jersey and his layover life. He complained to Mr. Poynor that the airline was cutting corners. He told a neighbor, Cindi Dressler, that the company started “messing” with his pay and pension.

“He found out that he could take his qualifications and go into a teaching position that would be more stable,” Ms. Dressler said. Her husband, Jeff Dressler, described Mr. Shaynak as “the smartest guy I ever met” and a natural teacher.

It is not clear why Mr. Shaynak left the airline in early 2008. He later told one student that the airline fired him for accusing it of safety lapses. He told another student he was laid off. He told others that he quit. That year, he bought a Mini Cooper, focusing his gear head energy on the new car.

A user named “sshaynak” joined a Mini Cooper online forum in March 2008. For his occupation, he wrote “I Quit!” He said he was moving to New York that June. He liked to drive with the car’s top down and windows up, so he could hear songs like “Back on the Chain Gang” by the Pretenders. He also liked Steely Dan. A car with a hard top, he wrote, was like “a great first date that ends with a handshake.”

Mr. Shaynak walked away from his home, letting it slip into foreclosure. He moved into an apartment in Washington Heights and started working as a substitute teacher that fall, before joining the sprawling faculty of Brooklyn Tech.

There, Mr. Shaynak reinvented himself as a modern-day Renaissance teacher. He made sure students learned, even if he had to raise the money himself to buy a 3-D printer, even if he had to build his own flight simulator. He let students use his classroom — more of a small hall — as a place to cut class, to come meet other friends, to talk about mining Bitcoin, to wreck a pretend plane on the simulator.

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Northport High School, where Mr. Shaynak was a member of the National Honor Society.CreditBryan Thomas for The New York Times

After his mother died, Mr. Shaynak moved back in with his father, commuting to Brooklyn from Long Island. The two were close, but fought often. There, Mr. Shaynak was the good neighbor, helping a boy with his computer, setting up a neighbor’s stereo. “Great guy,” said Michelle Foley, who cooked for them when Mr. Shaynak’s father had cancer.

Another neighbor, Frank Lingen, said Mr. Shaynak brought a younger woman with dark curly hair to the house during the summer of 2012. “They’d sit out and they would smoke cigarettes on the stoop,” Mr. Lingen said.

Mr. Shaynak’s father died in 2012, about a week after Hurricane Sandy. Mr. Shaynak, whose aerospace class persuaded him a month before to quit smoking, started up again, going through as many as two packs a day. He started going out at night more, one of his students said.

About this time, the authorities said, Mr. Shaynak was grooming girls, as young as 13, and targeting vulnerable ones. He is accused of exchanging 10,000 text messages with one student, whom he took to a nude beach. He had a four-month relationship with a former student, whom he took to a sex club, the authorities said.

Mr. Shaynak is accused of having sex with two girls after they turned 17, the age of consent in New York. In one case, he forced the girl, prosecutors said.

A former student who was close to Mr. Shaynak said he had long seemed to be in a “spiral of loneliness.” “Maybe with these students who looked up to him,” the student said, “who he cared about and who cared for him, he found what he was looking for, but he went about it inappropriately.”

The arrest has divided the school. Students who did not know Mr. Shaynak are horrified. But loyal students and alumni, one of whom set her mother up with Mr. Shaynak, she said, because she wanted him as a father, adopted “Free Shaynak” as their mantra online and plan to visit him in jail. They say Mr. Shaynak is innocent.

Because the thing about Mr. Shaynak? He talked to students like they were peers. He could talk a teenager through a breakup or a fear of dying. And he had the best stories. He was a pilot, after all.

In class, Mr. Shaynak seemed to live somewhat in those glory days. He reminisced about the time his student almost crashed the plane and he saved the day. He wore his ExpressJet uniform on occasion and posed for a picture alongside a female student wearing his pilot’s cap. When he signed up for Twitter, shortly after he started teaching at Brooklyn Tech, his name was CaptShayShay.

On a flying forum, Mr. Shaynak dispensed advice under a different name. There, he chose to be known as Captain Chaos.

Reporting was contributed by Al Baker, Russ Buettner, Elizabeth A. Harris and Jon Hurdle, and research by Susan C. Beachy.

A version of this article appears in print on , on Page A1 of the New York edition with the headline: A Teacher Accused of Abuse Seen to Have Never Grown Up. Order Reprints | Today’s Paper | Subscribe